1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory circuit, such as a flash memory, and particularly to a memory device capable of being switched between asynchronous read operations and clock-synchronous read operations.
2. Description of the Related Art
Semiconductor memory devices offer fast access times, and are utilized in computer systems for main memories and other memories of which fast access times are required. On the other hand, semiconductor memory devices are unsuited to storing large programs and large quantities of data; accordingly, hard disks are used in such large capacity memory applications. Of the various semiconductor memory devices, dynamic RAM is used principally for main memories. DRAM is a volatile memory that loses stored data when the power is shut off, and thus while suited to storing data or programs that are to be held in memory on a temporary basis, it is unsuited to storing the BIOS routine read out during computer boot-up, or other such programs.
Nonvolatile memories have attracted attention for the ability to hold stored data even when the power is shut off. In particular, flash memories that use nonvolatile memory, while presenting a certain limitation in terms of erase operations, are nonvolatile memories which, due to the basic operating principle of such memories, are capable of holding stored data even when power is shut off. Another advantage is faster access times than hard disks and other external memory devices. In computer systems offered in recent years, it has become common practice to store in flash memory the BIOS routine that is read out automatically at computer boot-up.
Operations executed at boot-up of a computer include automatic accessing of the flash memory in which the BIOS is stored and reading of the BIOS data at power-on; reading out the operating system (OS) stored on the hard disk; and placing this data in the dynamic RAM main memory. During subsequent execution of an application program, the OS placed in the main memory is serially read, and application program stored on the hard disk or other external memory device is executed. The dynamic RAM offered in recent years, such as SDRAM, are synchronous memories in which read operations are synchronized with the clock. Burst mode read operation, in which a continuous multiple-bit data stream is output synchronized with the clock, is an indispensable capability in terms of fast reads. Burst reads allow the OS to be read from the main memory at high speeds.
Conventional flash memory, on the other hand, is asynchronous memory, which has to wait for a given access time before reading out data stored at a given address in response to input of the address.
The OS generally consists of a smaller amount of data than does an ordinary application program; thus, as flash memory capacities have become greater, it has become possible to store the OS in flash memory, provided that there is sufficient data capacity to accommodate the OS. Storing the OS in flash memory obviates the need to transfer it to main memory from the hard disk, thereby significantly reducing the time required to boot up the computer. Since conventional flash memory is asynchronous, it is not possible for the system to perform the burst read operation on the flash memory storing the OS that are performed on the main memory. The slowness of read operations to flash memory diminishes the attractiveness of proposals to store the OS and other programs in flash memory.
Accordingly, it is an object of the present invention to provide a flash memory or similar nonvolatile memory circuit that is capable of clock-synchronous read operation.
It is another object of the present invention to provide a flash memory or similar nonvolatile memory circuit that is capable of burst read operation.
Yet another object of the present invention is to provide a flash memory or similar nonvolatile memory circuit that is capable of both clock-synchronous read operation and clock-asynchronous read operation. One other object of the present invention is to provide a flash memory that, viewed from the system end, is capable of conventional clock-asynchronous read operation, that is further capable of clock-synchronous read operation analogous to those for main memory, and that can be switched between the two modes as required.
Yet another object of the present invention is to provide a flash memory capable of being switched as appropriate between asynchronous read operation and burst read operation.